1. Field of the Invention
The present invention relates to a space solar cell, and more particularly to a space solar cell (solar cell for space application) such as a space silicon solar cell which has good electrical output power characteristics and is suitably used under the environment of outer space.
2. Description of the Related Art
A silicon solar cell is widely used as a solar cell that converts light energy to electric energy. Such a silicon solar cell is used also under the environment of outer space such as in an artificial satellite.
FIG. 14 shows an example of a conventional silicon solar cell. This is referred to as BSR (back surface reflector) structure, where an N.sup.+ -type diffusion layer 2 is formed on a light receiving surface located in a front surface of a 200 .mu.m thick P-type silicon substrate 1 by thermal diffusion of an N-type impurity ions for taking in the carriers generated by the light energy, and a light-receiving surface electrode 3 is formed in a comb-teeth shape on the N.sup.+ -type diffusion layer 2 for taking out the generated electricity. Further, the N.sup.+ -type diffusion layer 2 and the light-receiving surface electrode 3 are covered with an anti-reflection film 4 for reducing a surface reflection of incident light.
In addition, a BSR electrode 5 is formed on the back surface of the silicon substrate 1 for improving the amount of generated carriers by increasing an optical path length by reflecting a long-wavelength light that escapes away from the back surface of the solar cell. Further, a back surface electrode 6 is formed over an entire surface of the BSR electrode 5 for taking out the generated electricity. In the solar cell of this structure, a conversion efficiency is increased by allowing the light reaching the back surface of the silicon substrate 1 to be reflected by the BSR electrode 5 to take out an energy of the carries generated around the back surface effectively as an electric power.
A solar cell with further increased conversion efficiency is shown in FIG. 15 and is referred to as an NRS/BSF (non-reflective surface/back surface field) structure, where a light-receiving surface of a 100 .mu.m thick P-type silicon substrate 1 is formed into a non-reflection configuration 7 with numerous small inverted-pyramid recesses to reduce the surface reflection of solar light by multiple reflection. This is referred to as "NRS structure". Also, an N.sup.+ -type diffusion layer 2 is formed on a light receiving surface side of the P-type silicon substrate 1, and a front surface oxide film 8 is formed as a front surface dielectric layer on the N.sup.+ -type diffusion layer 2. A light-receiving surface electrode 3 having a comb-teeth shape is connected to the N.sup.+ -type diffusion layer 2 via openings formed in the oxide film 8. Further, the oxide film 8 and the light-receiving surface electrode 3 are covered with an anti-reflection film 4 for reducing the surface reflection of incident light.
Further, a P.sup.+ -type diffusion layer 9 is formed on a back surface side of the silicon substrate 1 for allowing the carries generated in the silicon substrate 1 to move towards the N.sup.+ -type diffusion layer (BSF structure). A back surface oxide film is formed as a back surface dielectric layer on the P.sup.+ -type diffusion layer 9. The BSR electrode 5 and the back surface electrode 6 are electrically connected to the P.sup.+ -type diffusion layer 9 via a plurality of openings 11 formed in the oxide film 10. An internal electric field is formed by the P.sup.+ -type diffusion layer 9, and the carriers generated near the back surface of the silicon substrate 1 are accelerated by this electric field, whereby recombination of the carriers is prevented and the energy of the carriers can be taken out effectively as an electric power. In this structure, the photosensitivity to a long-wavelength light increases to improve the conversion efficiency.
Compared with other materials, the solar cell utilizing a silicon substrate has a high conversion efficiency and is inexpensive, so that there has been a great demand for this type of a solar cell. Especially, in a space solar cell, a further improvement of output power is required in recent years and an improvement of an electric output power has been demanded. Therefore, an improvement of the output power must be achieved also in the solar cell having the above-mentioned structure.
Thus, in order to achieve an improvement in an output power of a solar cell having a BSR structure, a back surface dielectric layer may be formed on a back surface of a silicon substrate to reduce the recombination, at the back surface, of carriers generated by the light energy, whereby the electric output power can be improved. Such a solar cell is disclosed, for example, in Japanese Unexamined Patent Publications No. HEI 04(1992)-274374 and No. HEI 06(1994)-169096.
On the other hand, in the solar cell having an NRS/BSF structure, the back surface dielectric layer is already formed on the back surface of the silicon substrate. With respect to increasing the conversion efficiency of the silicon solar cell, "Conference Record" 21th IEEE, Photovoltaic Specialists Conference, Florida, May 1990, pp. 333-335, for example, proposes a technique in which the substrate includes a plurality of locally-formed P.sup.+ layers and a silicon oxide film is used as the back dielectric layer. Also, Japanese Unexamined Patent Publication No. HEI 04(1992)-15963 discloses asolar cell inwhich a special arrangement of diffusion layers is provided to increase the conversion efficiency.
In recent years, in addition to the increase of the conversion efficiency of a solar cell, there is also a demand for a space solar cell having good electric output power characteristics in which the radiation hardness is considered so that the solar cell can be used under the environment of outer space.
The present invention has been made in view of these circumstances, and the purpose thereof is to provide a space solar cell that can be suitably used under the environment of outer space by defining an aperture ratio in the back surface dielectric layer, thereby to improve the electric output power characteristics.
The inventors of the present invention have found out that the loss in the electric power due to series resistance can be reduced and the electric output power characteristics can be improved by defining an aperture ratio (area ratio) to be within the range from 0.25 to 30%, the aperture ratio being a ratio of the area occupied by a plurality of openings formed in the back surface dielectric layer for establishing an electrical connection between the semiconductor substrate and the back surface electrode, relative to the area of the back surface of the semiconductor substrate. This finding has lead to the present invention.